ABSTRACT In the past decade, T lymphocyte-based cancer therapy has achieved significant success. However, patients with advanced cancer are very resistant to cancer immunotherapy. The difficulties to induce T cells that can survive and persist in the tumor microenvironment (TME) have limited the potential of T cell-based cancer therapy. The tumor-induced expansion of suppressor cells such as regulatory T cells (Tregs) and myeloid- derived suppressor cells (MDSCs) also limits the efficacy of T cells. Thus, enhancing T cell capacity of survival in the TME and targeting suppressor cells could improve current immunotherapies. IL-27 is an anti-inflammatory cytokine that has shown anti-tumor activity. Recently, we have obtained compelling evidence that IL-27 may be used as a novel therapeutic for cancer. First, IL-27 induces a new subset of Th1/Tc1 effector T cells that we term effector stem cells that can survive better in TME. Second, IL- 27 treatment of mice leads to a dramatic reduction of regulatory T cells. However, we also found that IL-27 induced the expansion of MDSCs. We hypothesize that IL-27 signaling directly induces the expansion of MDSCs and targeting MDSC can improve IL-27-based cancer immunotherapy. To test this hypothesis, we will first characterize the functions of IL-27-induced MDSCs and determine if MDSC-depletion enhances IL-27- mediated anti-tumor immunity. Additionally, we will determine if IL-27 directly induces the expansion of MDSCs via Stat3-Cyclin D1 axis in myeloid lineage. We will generate mice with Stat3 conditional deletion in myeloid lineage or pharmacologically inhibit Stat3 activation to determine if AAV-IL-27-mediated expansion of MDSCs and tumor immunity are affected. The proposed studies will not only reveal new insights of the anti-tumor activity of IL-27, but also lead to the discovery of new therapeutic candidates that can work synergistically with IL-27 for cancer treatment.